This invention relates to devices for controlling polarization of incident optical signals and, more particularly, to devices which permit endless or reset-free operation.
Optical signals in standard, non-polarization preserving optical fibre-based communication systems experience random changes in polarization state from one end of the fibre to the other due to fibre birefringence induced by temperature fluctuations and physical stresses on the fibres. Random polarization changes are evidenced at the output end as polarization mode dispersion (PMD) fluctuations.
In order to correct the polarization state of lightwave signals emerging from the optical fibre transformers have been developed to transform the fibre output polarization into the prescribed polarization state for applications such as heterodyne detection and interferometric signal processing. Conventional polarization transformers provide compensation but require a reset cycle when their operating range is exceeded. Unfortunately, reset cycles give rise to periods of unacceptable data loss. Endless polarization transformers provide continuous control of the polarization state over an infinite range of polarization compensation.
Endless polarization transformers have been developed using cascaded polarization transformers having a limited transformation range such as fibre squeezers and electrooptic devices using lithium niobate or PLZT. While these cascaded devices permit truly endless (reset free) operation, individual elements within the devices still require occasional reset cycles. Although the reset cycles can be performed without affecting the overall polarization transformation (quasi-endless polarization control), these devices generally fail to permit polarization control during reset cycles. Moreover, they require sophisticated and even computer controlled drive algorithms for proper operation.
Fibre squeezers mechanically induce birefringence in the fibre axes to cause retardation between the two orthogonal modes perpendicular and parallel to the direction of pressure. U.S. Pat. No. 5,561,726 in the name of Yao, describes a system that utilizes a rotatable fibre clamp to supply the necessary retardation and optical axis orientation. Although this device can be used for fixed wavelength and temperature and polarization it cannot be used to control real time polarization fluctuation in transmission fibres, because it requires mechanical movement for its control.
In the past, a reset-free, endless polarization transformer was demonstrated performing general polarization transformations from any arbitrarily varying optical input polarization into any arbitrarily output polarization by producing adjustable elliptical birefringence of constant total phase retardation in a single-mode waveguide. See U.S. Pat. No. 4,966,431 issued to Heismann on Oct. 30, 1990. A particular transformation is obtained by adjusting the azimuth of linear birefringence and the ratio of linear to circular birefringence. In its integrated-optic realization, the endless polarization transformer includes at least one cascadable transformer section comprising cascaded first and second TE TM mode converters. Phase shifting (TE/TM) is performed in a section between the mode converters, in a section following the mode converters, or both between and following the mode converters. All sections are formed over a birefringent waveguide capable of supporting propagation of TE and TM optical signal modes. While the recent endless, reset-free polarization transformer is cascadable and affords simplicity of design and operation over prior art devices, it cannot be overlooked that this polarization transformer has a relatively narrow optical bandwidth at wavelengths of interest less than 1 nm at 1.55 xcexcm and permits only limited tunability over a small wavelength range approximately 10 nm.
Heismann in U.S. Pat. No. 5,212,743 entitled Automatic Polarization Controller Having Broadband Reset-Free Operation, incorporated herein by reference discloses a wide optical bandwidth and broad wavelength tuning range achieved in a reset-free, optical, automatic polarization controller by combining three controllable fractional wave elements in cascade and further by controlling the orientations of both outermost fractional wave elements to differ by a prescribed angular amount which is maintained substantially constant. Synchronous control of both outermost fractional wave elements maintains the prescribed angular difference constant during operation of the polarization controller.
In the embodiments described by Heismann, the three fractional wave elements are provided in the form of an endlessly rotatable half-wave element and two synchronously rotatable quarter-wave elements wherein the half-wave element is placed between the quarter-wave elements. Each fractional wave element varies the orientation of retardance along its optical wavepath and introduces a specified phase retardation. Embodiments of the polarization controller are realized using either distributed bulk optic devices or integrated electro-optic waveguide devices. Rotation of the elements is afforded by a feedback control circuit which monitors the output optical polarization and derives appropriate electrical drive signals to achieve the proper rotation of the elements. Although the device taught by Heismann appears to achieve its intended function in many instances it does not provide a precise enough, hence an ideal-enough quarter or half waveplate. For example, in practice, it has been found that controllers of the type taught by Heismann are very difficult to manufacture with enough precision with materials that are uniform enough in their response, to provide glitchless operation. For example, misalignment of the electrodes on the birefringent material, or non-uniformity in the birefringent material will negatively affect the performance of the device.
In contrast, the instant invention provides a means for attaining superior performance by providing means to compensate for such aberrations. Essentially a greater resolution is afforded and by achieving this, imperfect regions, deviations in the birefringent material, or mis-alignment can be compensated for to achieve a substantially fixed retardance within a waveplate section of, for example a typical three-section controller.
It is an object of this invention to provide an automatic polarization controller having broadband operation wherein the quarter waveplates and half the waveplate are nearly ideal.
It is a further object of the invention to provide an inexpensive, highly responsive device for controlling polarization of an input beam of light having varying polarization states.
It is a further object of this invention to provide a controllable quarter waveplate or half waveplate for use, for example in a polarization control circuit.
In accordance with the invention, an electro-optic waveplate for changing the state of polarization of light passing therethrough while providing a substantially constant birefringence when the principal birefringent axes of the electro-optic waveplate are rotated is provided, comprising:
a birefringent material having two principal orthogonal birefringent axes that are rotable in the presence of a suitably applied voltages, the birefringent material having a first end and a second end and having a longitudinal axis of length L defined therebetween;
means for controllably providing at least four related different voltages along sequential or contiguous regions along the length L for providing a controllable and varying electric field along the length L, such that retardance of the waveplate of the length L remains substantially constant while the birefringent axes of the electro-optic waveplate are rotated by varying at least the voltages, wherein the at least four different voltages have a phase relationship or a phase and magnitude relationship therebetween.
In accordance with the invention, there is further provided, a quarter waveplate or a half waveplate comprising a first pair of electrodes spaced apart along a block of birefringent material serving as voltage terminals to provide two different and related electric fields through the material simultaneously in response to two different applied voltages;
a second pair of electrodes spaced apart along the block of birefringent material serving as voltage terminals to provide two other electric fields through the material simultaneously in response to two other applied voltages; and
means for applying voltages of the form
V1=Vs1 sin(xcex8)+VC1 cos(xcex8)+VT1
V2=VS2 sin(xcex8)+VC2 cos(xcex8O)+VT2
xe2x80x83to the first electrodes, and
voltages of the form:
V1xe2x80x2=VS1xe2x80x2 sin(xcex8+xcex1)+VC1xe2x80x2 cos(xcex8+xcex1)+VT1xe2x80x2
V2xe2x80x2=VS2xe2x80x2 sin(xcex8+xcex1)+VC2xe2x80x2 cos(xcex8+xcex1)+VT2xe2x80x2
xe2x80x83to the second electrodes,
where 0 less than xcex1 less than 360xc2x0 and where xcex8 can be any angle and endlessly varying.
In accordance with the invention there is further provided, a polarization controller comprising electrically controllable waveplates arranged in a predetermined spatial relationship having a same longitudinal axis of propagation to allow light launched into one of the waveplates to propagate through the other of the waveplates, at least one of the waveplates being formed of plural pairs of electrodes spaced across a birefringent material to provide at least four different electric fields along the axis of propagation through the material to light propagated therein in the presence of suitably applied voltages; and,
means for providing the suitably applied voltages to yield the at least four different electric fields to light passing through the birefringent material, such that a substantially quarter or half wavelength of retardance will result for light passing therethrough wherein the at least four different electric fields are of a magnitude and phase to ensure a substantially constant retardance through said one waveplate as birefringent axes of the waveplate are rotated.
In accordance with another aspect of the invention a method of providing a near-ideal quarter or half waveplate is provided comprising the steps of:
launching a signal into a block of electro-optical material having a length L;
providing four different voltages to the block of electro-optical material that will yield four different fields therethrough along the length L, where the voltages have a magnitude or phase relationship therebetween; and
ensuring that product of the length and voltages is sufficient to a substantially constant retardance along the length L in the presence of the two different fields.
In accordance with the invention, a polarization transformer is provided for controlling the polarization and phase of an optical signal comprising:
a block of electro-optical material having a plurality of electrode pairs thereon for applying quadrature voltages thereto, each pair of terminals having a third common terminal disposed therebetween, said block of birefringent material, in the presence of an applied voltage for forming a near-ideal controllable waveplate, a first plurality of the plurality of pairs of electrodes for inducing a phase retardation of an optical signal passing through the block of substantially about xcfx80/2 radians and forming a first quarter waveplate; a second plurality of the pairs of electrodes for inducing a phase retardation of an optical signal passing through the block of substantially about xcfx80 radians forming a first half waveplate; and, a third plurality of the plurality of pairs of electrodes for inducing a phase retardation of an optical signal passing through the block of substantially about xcfx80/2 radians and forming a second quarter waveplate. The three above-mentioned waveplates need not lie in a particular order.